A well-known example of a device as defined in the preceding paragraph is a device for disinfecting water, in which the element which is arranged inside the housing is a source such as a lamp for emitting ultraviolet light, in particular of the type commonly referred to as UV-C. Exposing infected water to the ultraviolet light has a purifying effect on the water on the basis of the fact that the UV-C light is capable of sterilizing germs. For domestic applications, the ultraviolet source is usually enclosed in a non-opaque holding structure, enabling the desired exposure of the water to the source.
For sufficient germicidal action, the ultraviolet disinfecting device has to produce a certain ultraviolet dose, expressed in J/m2. The dose is given by the irradiance (W/m2) multiplied by a residence time (s) of the bacteria in the device. The residence time as mentioned is determined by the flow paths of the water, and the irradiance level by the type of ultraviolet source which is applied.
The use of flow straightening means in a water disinfecting device is known from EP 1837309, for example. The known flow straightening means comprise a cylinder-shaped element which is provided with a number of perforated sheets, wherein each of the sheets comprises a grid of lamellae and star-shaped openings. The perforated sheets are formed by subjecting a basic sheet to a punching process, and subjecting the perforated sheet which is obtained to a stretching process. The lamellae and the openings are arranged in a regular pattern, and the sheets are arranged in parallel such as to form a stack, in a specific way, namely such that the patterns of the lamellae and the openings of adjacent sheets have a defined shifted arrangement.
When designing a water disinfecting device, it has to be kept in mind that the irradiance of the ultraviolet source decays linearly, or, depending on the extent to which absorption takes place, linearly and exponentially with the radial distance to the source. In view of this fact, in order to achieve a desired effectiveness of the device, it is advantageous if measures are taken to ensure that radial mixing of the water elements takes place. By providing for radial mixing it is intended that all water elements pass the ultraviolet source on a very short distance at some point. Irradiance levels close to the source are so large that a short exposure of the bacteria in the vicinity of the source is sufficient to eliminate them.
It follows from the foregoing that radial mixing is a way of achieving a required dose in a limited exposure time or compact design. However, when radial mixing is applied, several aspects need to be taken into account. In the first place, a mixing element covers the ultraviolet source, at least to a certain extent, and thereby leads to a decrease in irradiation level. Thus, applying a mixing element is only useful when the gain in dose level as a result of an improved flow profile overcomes the losses as a result of a lower irradiance level. Transparent mixing elements can also be used, but such means must be resistant to ultraviolet light and may not be too fragile (glass). It is possible to use upstream static mixing elements which do not have a decreasing effect on the irradiation level, but in that case, the mixing level decays along a length of the device. Consequently, in that case, the highest radial mixing occurs in an inlet area of the device, whereas mixing, and specifically radial mixing, in a downstream area may be minimal or absent. In the second place, it is a fact that water disinfecting devices are mostly used in areas where tap pressure is low. Adding a mixing element may lead to unwanted high pressure drops.
Another problem that is encountered in the field of water disinfecting devices is that so-called short-cuts may be present. Short-cuts are flow paths leading directly from the inlet to the outlet. Bacteria following these short-cuts have very short residence times. Especially when the short-cuts prevail at an outer radius of a section where the ultraviolet source is arranged, remote from the ultraviolet source, at which location irradiance levels are lowest, very low dose levels result. Short-cuts are generally caused by certain inflow conditions and/or inappropriate mixing.